Our invention relates to piezoelectric crystals, and particularly to improved electrodes for piezoelectric crystals and a method of manufacturing such improved electrodes.
Piezoelectric crystals for radio and electronic circuits require electrodes for connection to the circuits. Such electrodes are provided by an initial evaporation of a high purity metal, such as gold, silver or aluminum, on each crystal after it has been lapped and etched to an appropriate unelectroded frequency. This initial metal evaporation is called baseplating. The areas of metal supplied by the evaporation are determined by precision photo-etched masks which define the resonator and conductor patterns on each face of the crystal plate or blank. The crystal blank is held in place by a fixture located in a chamber. The fixture and the crystal blank are sandwiched between the photoetched masks which define the correct electrode pattern on each face of the crystal blank. A typical baseplate mask may accommodate an array of 20 to 40 crystals. A container of the baseplate metal is also placed in the chamber. The chamber is then sealed, partially evacuated, and vented or purged with an atmosphere of dry nitrogen to dispel contaminants and impurities. After this, the chamber is pumped down to leave a high vacuum. The container of baseplate metal is then heated to evaporate the metal on the unmasked crystal surfaces. When the evaporation is complete, the correct resonator and conductor baseplate patterns or electrodes are provided on the appropriate surfaces of the crystal blank in accordance with the mask patterns.
If silver is the metal evaporated to provide the baseplate, the crystal is mounted in a suitable holder following application of the baseplate. Suitable leads are cemented to the baseplate electrodes, using conductive cement. With leads provided, the baseplate is electroplated with nickel to a predetermined frequency based on the final crystal frequency and the frequency limitations of the final frequency adjustment method. As known in the art, the nickel electroplate tends to metallurgically stabilize the silver baseplate electrodes and conductors. The nickel electroplate thus reduces the effects of contamination and aging during the time interval following the electroplating and before the final frequency adjustment. The nickel electroplate also stabilizes frequency shifts which occur during sealing of the crystal in a suitable can, and further reduces effects of aging after the crystal is sealed in the can. In addition, the nickel electroplate serves to adjust the final frequency of the crystal close enough so that conventional automated final frequency adjustments can be made by further evaporating silver through a mask over the nickel electroplate. This mask is usually somewhat smaller than the nickel electroplated surfaces. In the manufacture of coupled-dual crystals, the prior art, automated active frequency adjustment is capable of adjusting the final frequencies a relatively small amount. This adjustment is limited because of the limitations of the automated measuring techniques. For example, see the article entitled "An Equivalent Circuit Approach to the Design and Analysis of Monolithic Crystal Filters" by R. C. Rennick; IEEE Trans. on Sonics and Ultrasonics, Vol. SU-20, No. 4, pp. 347-354, Oct. 1973. This plating and measuring process have been used for coupled-dual crystals that operate in the range from about 5 megahertz to 25 or 30 megahertz.
However, in the case of higher frequency coupled-dual crystals, such as in the range of 45 megahertz, third overtone crystals must often be used, particularly in first IF filter applications for radios. At such frequencies, IF bandwidth requirements and impedance level limitations often lead to 45 Megahertz third overtone coupled-dual crystals which require very small mass loading on the crystal plate itself. If gold or silver is used to provide the initial metalization onto the crystal plate, such plating or metalization is often so thin that the crystal will not oscillate. Therefore, we have found that it is highly desirable, if not essential, that a relatively light metal such as aluminum be used to provide the initial baseplate on the crystal. In such a case, the crystal resonators are adjusted to final frequency by evaporating a metal such as silver or gold through a mask whose opening has an area smaller than the resonator electrode to be adjusted to provide a fairly small area. However, this arrangement produces a crystal which needs or requires a relatively large termination resistance when used in a filter application. Although such a crystal is acceptable for some applications, such a requirement can cause radiation and matching problems in a radio receiver.
Where the bandwidth required of a crystal resonator serving as an IF filter increases, as it may in certain radio applications, the terminating impedance becomes even larger, and the spacing between the crystal electrodes becomes impractically or unacceptably small, unless photolithographic techniques are used. Therefore, a fundamental frequency coupled-dual crystal must be used. At the high frequencies, such as 45 megahertz, although the required plateback can be larger, the crystal itself is so thin that only very light platings or mass loadings can be used.
In view of the problems mentioned above, it is frequently desirable to use aluminum to baseplate the crystal, followed by silver to provide a final frequency adjustment. However, this causes the crystal frequency to vary in a very irregular manner after the crystal is sealed in its can or container.